In a communication network, the clock is vital to the quality of network services. If the clock of a network is abnormal, service data pointer justification occurs, or even worse, the whole network breaks down.
Generally, the network uses one or more external clock sources to provide a clock reference for each node. Each network node traces an external clock source according to the tracing relation planned in a specific way, and switches the tracing relation when the network status changes. The clock tracing relation needs to fulfill one important principle: the clock tracing relation cannot be looped anytime. If the tracing relation is looped, for example, node A traces B, and B traces A, the network service deteriorates and fails shortly thereafter.
The traditional transport network performs clock tracing through the Synchronization Status Message (SSM) protocol. The SSM is adapted to transmit the quality level of timing signals in a synchronization timing link. Therefore, the node in the Synchronous Digital Hierarchy (SDH) network and the synchronized network obtains the information about the upstream clock by reading the SSM, operates the clock of the local node accordingly, for example, tracing, or switching for holding, and transmits the node synchronization information to the downstream node.
FIGS. 1A and 1B show a functional structure of a clock of an SDH device compliant with the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) G.783 recommendations. In FIGS. 1A and 1B, T1 is the input interface of the Level-N Synchronous Transfer Module (STM-N), and receives signals from the STM-N line; T2 is the input interface of the Plesiochronous Digital Hierarchy (PDH), and receives signals from a PDH tributary; T3 is an external synchronization input interface, and receives external timing input reference signals; Synchronous Equipment Timing Generator (SETG) is an SDH Equipment Clock (SEC); T4 is an external synchronization output interface, and its timing output may be exported by the STM-N line directly or from the SETG; and T0 is an internal timing interface.
In FIGS. 1A and 1B , the orde of priority for selecting the timing reference clock signals (from high to low) by a selector is as follows:                1. Manual forced command, for example, forced holding, or forced switching;        2. Timing signal failure, for example, LOS, AIS or OOF (LOF);        3. SSM quality level; and        4. Preset priority.        
The traditional transmission topology is simple, namely, is mainly a ring or a chain. With the development of transport networks, the transport network topology evolves to a mesh network topology. As a result, the traditional SSM protocol is hardly adaptive to the development of transport networks due to functional limitation.
In a network, the corresponding reference source priority table needs to be configured for all network elements according to the clock tracing conditions. Therefore, the clock tracing needs to be well planned through manual configuration at the beginning of network construction; that is, the corresponding source priority list needs to be configured manually to prevent clock interlocking and prevent a high-stratum clock from tracing a low-stratum clock anyway. Meanwhile, two reference sources are configured for each network element in the network for the purpose of mutual protection.
However, the prior art is vulnerable to clock tracing loops and unable to prevent a high-stratum clock from tracing a low-stratum clock, and tends to make a network element unable to trace the source.